Electrical braking system for electric motor-driven vehicles

ABSTRACT

An electrical braking system is provided for a vehicle driven by an electric motor, having a field winding arranged so that the vehicle is driven in a direction in accordance with the direction of direct current flowing through the filed winding. The system, which is adapted to be carried by such a vehicle, includes a movable brake member adapted to be mounted to the vehicle for movement between a normal first position and a second position. Circuit means, which may include a variable resistor, serves to vary the amount of direct current applied to the field winding in dependence upon the relative position of the brake member between the first and second positions. An electrical sensing circuit is utilized to sense whether the vehicle is traveling in a forward or reverse direction. A current direction control circuit serves to control the direction of direct current through the field winding so as to flow in a direction opposite to that required for driving a vehicle in the sensed direction. In this manner, the electrical motor is electrically braked to thereby retard motion of the vehicle in the sensed direction substantially in proportion to the relative position of the brake member.

United States Patent [72] inventor Dllkl]. S0111 Primary Examiner-GrisL. Rader Norristown, Pl. Assislam Examineri(. L. Crosson [21] Appl. No.33,181 AttrneyYount and Tarolli [22} Filed Apr. 30, 1970 [45] PatentedAug. 3, 1971 73 Assi ee Eaton Yale 8: T l 1 Sn Ckvehnd 8 ABSTRACT: Anelectrical braking system is provided for a vehicle driven by anelectric motor, having a field winding arranged so that the vehicle isdriven in a direction in ac- 54 CT IC BRAKING SYSTEM FOR ELECTRICcordance with the direction of direct current flowing throughMOTORDRIVEN VE the filed winding. The system, which is adapted to becarried 19 Cm 4 m ing It by such a vehicle, includes a movable brakemember adapted 52 Us CI to be mounted to the vehicle for movementbetween a normal 318/17, first Position and a Second Position Circuitmeans, which may 51 n C 18/258 318/371 318/373 include a variableresistor, serves to vary the amount of direct [50] Field ..H02k 17/14current applied to the field winding in dependence upon the I 1 relativeposition of the brake member between the first and 187/9 182/84 secondpositions. An electrical sensing circuit is utilized to [56] Rd CMsienesztyvhetiier the ve iclet is traveling in a iprward or reveti'selir 1011. curren lrec ion con r0 ClI'CLll serves 0 con ro UNITED STATESPATENTS the direction of direct current through the field winding so as3,213,343 /1 511 1 1 313/373 to flow in a direction opposite to thatrequired for driving a 3,297,930 1/1967 Payne 3 l8/269 vehicle in thesensed direction. In this manner, the electrical 3,335,351 3/1967 M 1111 7 motor is electrically braked to thereby retard motion of the3,344,328 1 7 M rri 1 /269 vehicle in the sensed direction substantiallyin proportion to 3,463,991 8/1969 Yuminaka et al 318/258 the relativeposition of he brake m m r- H, l 5 I M 1?; ct 17- PS M r I am aw-aGil-'4 new q s. l .mm. M a,

32 8 t, CK5-2 Li ELECTRICAL BRAKING SYSTEM FOR ELECTRIC MOTOR-DRIVENVEHICLES This invention is related to the art of braking and, moreparticularly, to a system for electrically braking electric-poweredvehicles, such as industrial lift trucks or order pickers.

The invention is particularly applicable in conjunction withelectrically powered order pickers, lift trucks, or the like; however,it will be appreciated that the invention has broader applications andmay be used in conjunction with electrically braking various types ofelectric motor powered vehicles.

Industrial vehicles, such as lift trucks and order pickers, arefrequently driven by electric motors. Such vehicles may be mechanicallybraked or electrically braked, or a combination of both. Typically, themechanical braking action is an on-off operation; to wit, a brake memberis spring biased into braking engagement with one of the vehicle'swheels and when released, as with a solenoid, the vehicles wheel is freeto be driven by the motor. Electrical brakes are also on-off brakes, andnormally involve dynamic braking by plugging the field coil of thedriving motor; to wit, applying full rated direct current to the fieldwinding in a direction opposite to that required for driving the vehiclein the direction it is traveling when the electric brake is actuated.Consequently, both mechanical and electrical onoff brakes provide abruptbraking forces.

An order picker is generally similar to a lift truck in that it includesa lift which is adjustably movable in a vertical direction between alower floor position and an upper posi tion. In an order picker,however, an operators compartment is located on the lift itself,together with controls for controlling the vehicle's speed anddirection, as well as the vertical position of the lift. Consequently,the operator may drive the vehicle to a desired location and, withoutleaving the operators compartment, operate the lift controls to aparticular height to fill an order from an elevated bin. If the vehicleis driven while the lift is in a raised position, then it is desirablethat the vehicle be provided with a braking system that provides asmooth rate of deceleration. The typical on-off electrical andmechanical brakes provide abrupt rates of deceleration. If a mechanicalservice brake be devised for the vehicle to obtain a smoothdeceleration, the brake would require complex mechanical linkagesextending between the lift and the vehicle's wheels to permit brakingoperation at various positions of the lift.

The present invention provides electrical circuitry for reversing theflow of direct current to the field winding of an electric motor in sucha vehicle to obtain dynamic braking. So that the braking action issmooth and not abrupt, reverse current is varied in accordance with theposition of a brake member. With this invention, the electrical brakingcircuitry utilizes much of the components normally required to drive thevehicle; to wit, a direct current power source, pulse control circuitry,motor field windings and field winding reversing circuitry.

The present invention contemplates a vehicle driven by an electricmotor. The motor should include a field winding arranged so that thevehicle is driven in a direction in ac cordance with the direction ofdirect current flowing through the field winding.

In accordance with the present invention, the electrical braking systemis adapted to be carried by such a vehicle and includes a movable brakemember which is adapted to be mounted to the vehicle for movementbetween a normal first position and a second position. Circuit means areprovided for varying the amount of direct current applied to the fieldwinding dependent upon the relative position of the brake member betweenthe first and second positions. Sensing circuitry serves todeterminewhether the vehicle is traveling either in a first or a secondof two opposite directions. Also. current direction control circuitryserves to control the direction of the direct current through thefielding winding so as to flow in a direction opposite to that fordriving the vehicle in the sensed direction. In this manner, theelectrical motor is electrically braked to thereby retard motion of thevehicle in the sensed direction substantially in proportion to therelative position of the brake member.

In accordance with a more limited aspect of the present in vention, thecurrent varying circuitry includes a variable resistor having a wiperarm mechanically coupled to the brake member for movement therewith.

Further, in accordance with the present invention, circuitry is providedfor normally rendering the variable resistor deactivated and foractivating the variable resistor when the brake member is displaced fromits normal first position toward the second position.

Still further in accordance with the present invention, circuitry isprovided for deactivating the current direction control circuitry uponthe concurrent conditions that the vehicle is not moving, and the brakemember is displaced from its first position toward its second position.

The invention also contemplates that the vehicle may be provided with apulse control circuit for controlling application of direct currentpulses to the electric motors field wind ing. In such case, theinvention may also include a pulse stretching circuit for increasing thewidth of the current pulses prior to application to the field winding tothereby obtain greater electrical braking forces for a given pulsefrequency.

The primary object of the present invention is to provide an electricalbraking system for obtaining smooth deceleration of an electric motorpowered vehicle.

A further object of the present invention is to provide an electricalbraking system having a movable brake member and control circuitry fordecelerating both the electric motor and the vehicle powered thereby inaccordance with the position of the brake member.

Another object of the present invention is to provide an electricalbraking system which utilizes the electric motor of an electric motordriven vehicle for decelerating the vehicle.

A still further object of the present invention is to provide anelectrical braking system for an electric motor driven vehicle having anelectronic speed control circuit, and wherein the speed control circuitis utilized in electrically braking the vehi cle.

A still further object of the present invention is to provide anelectrical braking system particularly applicable for use inelectrically braking an order picker vehicle.

The foregoing and other objects and advantages of the invention will bemore readily apparent in light of the following description of theinvention given in conjunction with the accompanying patent drawingswherein:

FIG. 1 is a perspective view illustrating an order picker vehicle;

FIG. 2 is a view taken along line 2-2 in FIG. 1 and illustrating a brakemember mounted to the vehicle for movement between a first position anda second position; and,

FIGS. 3 and 3a, taken together, constitute a combined block-schematicdiagram illustrating the electrical circuitry employed in the preferredembodiment of the invention.

GENERAL DESCRIPTION Referring now to the drawings wherein the showingsare for purposes of illustrating the preferred embodiment of theinvention only, and not for the purpose of limiting same, FIG. Iillustrates an order picker vehicle A having a lift L which isadjustably movable in a vertical direction between a lowermost floorposition, shown by the solid lines in FIG. I, and an upper position,shown by the dotted lines. The vehicle may have conventional steeringmechanisms, but as shown in the drawings, includes steering circuitryfor following a path defined by an alternating current energized cable10. The steering forms no part of the present invention and, hence, willnot be described in greater detail hereinafter. The lift L is raised orlowered by a suitable mechanism (not shown). A suitable handle grip 16serves to control the speed of the electric motor which drives thevehicle. The operator stands on a platform 18.

In accordance with the present invention, a brake member is mounted onplatform 18 of lift L for movement between a normal first position, asshown by solid lines in FIG. 2, and a second position, as shown by thedotted lines. Brake member 20 is normally held in its first position asby use of compression springs 22. A normally Open limit switch LS4 issuitably mounted to the structure so that it is actuated to its closedcondition once brake member 20 is slightly displaced from its normalfirst position toward its second position. Also, a normally closed limitswitch LS-Z is suitably mounted to lift L so as to be actuated to itsopen position when brake member 20 is displaced to its second position.The purpose of limit switches LS-l, LS-Z will become more readilyapparent from the description which follows, taken in conjunction withFIGS. 3 and 3a. Brake member 20 is mechanically coupled to the wiper arm26 of a variable resistor, or potentiometer, 28 so as to vary theresistance thereof as the brake member 20 is displaced from its firstposition toward its second position.

Reference is now made to FIGS. 3 and 3a, which provides a detailedillustration of the electrical braking system constructed in accordancewith the present invention. As discussed hereinbefore, the inventioncontemplates that a vehicle be driven by an electric motor, such asmotor M, having a shunt field winding 30 and an armature 32 for drivingthe vehicles wheels, including a wheel generally designated as wheel 34.The vehicle is provided with a mechanical brake, such as brake 36,having a brake shoe 38 normally spring biased into engagement with wheel34, as with a spring 40, to prevent vehicle movement. Brake 36 isillustrative of the typical on-off mechanical brake used in such avehicle. Vehicles of this nature are frequently provided with a batteryB made up of several small batteries so as to obtain B+, B-, C+ and C-voltages. The and B- voltage levels may respectively be plus and minus18 volts, and the C+ and C- voltages may be respectively plus and minus12 volts. The B-lvoltage is typically coupled to the arma ure 32 andthrough a pulse control circuit PC to the B- voltage supply source. Thepulse control circuit PC serves to provide current pulses ofa givenpolarity, magnitude, and pulse width to the armature 32 at a frequencydependent upon the value of a resistor connected between the B- voltagesource and a control input of the pulse control circuit. The resistorconnected to the control input of circuit PC takes the form of variableresistor 42, which normally serves as an accelerator control forcontrolling vehicle velocity, Pulse control circuit PC may take variousforms and generally includes at least one silicon controlled rectifier44, shown with dotted lines in FIG. 30, together with appropriatecontrol circuitry for controlling the frequency of firing pulses appliedthereto. Preferably, the pulse control circuit takes the form of anelectronic pulse control circuit available from General Electric Companyas Model No. C185.

In accordance with the present invention, the electrical braking systemincludes a direction sensor circuit D5, which provides an output signalhaving a polarity indicative of vehicle direction and a magnitudeindicative of vehicle speed. The output circuit of sensor circuit DS iscoupled to a current direction control circuit CD, which, upon closureof limit switch LS-I by brake member 20 (see FIG. 2), serves to controlthe direction of the direct current pulses taken from the output circuitof pulse control circuit PC through field winding 30 so that thesepulses flow in a direction opposite to that required for driving thevehicle in the direction sensed by circuit DS. In this manner, thereverse current flowing through field winding 30 serves to electricallybrake the electric motor and, hence, the vehicle driven by the motor.The rate of deceleration is dependent on the relative position of brakemember 20. Having now presented a brief description of the invention,attention is directed to the following detailed description of theinvention.

DIRECTION SENSOR CIRCUITRY The direction-sensing circuit DS serves thepurpose of pruhtllng an output signal indicative of the direction ofvehicle travel, as well as vehicle velocity. This circuitry has itsinput coupled to the output of a DC tachometer 50 which is suitablycoupled to the vehicle, such as to wheel 34, so as to provide an outputsignal having a polarity indicative of vehicle travel direction and amagnitude indicative of vehicle velocity. The output circuit oftachometer 50 is coupled through re sistors 52 and 54 to one input of anoperational amplifier 56. The operational amplifier has a feedbackresistor 58 coupled between its output circuit and its input circuit. Asecond input circuit of operational amplifier 56 is coupled to ground. Apair of diodes 59 and 60 are connected together in parallel between thejunction of resistors 52 and 54 and groundv As shown in FIG. 3, thediodes are oppositely poled with respect to each other. A capacitor 612is connected in parallel with diodes 58 and 60 and a resistor 64connects the output circuit of operational amplifier 56 with the inputof current direction control circuit CD. If the vehicle is traveling ina forward direction, then the output signal of direction sensing circuitD5 will be of negative polarity, and, conversely, if the vehicle istraveling in an opposite reverse direction the output signal will be ofpositive polarity.

CURRENT DIRECTION CONTROL CIRCUIT The purpose of the current directioncontrol circuit CD is to control the direction of current flow throughfield winding 30 during the electrical braking operation. If the vehicleis traveling in the reverse direction then circuit CD will cause thefield current to flow through field winding 30 in a forward direction,as indicated by the arrow F adjacent winding 30 in FIG. 3a. Conversely,if the vehicle is traveling in a forward direction, then circuit CD willcause the field current to flow through winding 30 in a reversedirection, as indicated by arrow R.

The current direction control circuit CD includes a forward controlcircuit 68 and a reverse current control circuit 66. Circuit 66 includesa field current control means including relay coil CR4-C and a circuitincluding NPN transistors 70 and 72 for controlling energization ofrelay coil CR4-C. Similarly, forward control circuit 68 includes a fieldcurrent control means in the form of relay coil CR3-C, together withcircuitry including PNP transistor 74 and NPN transistors 76 and 78 forcontrolling energization of relay coil CR3-C.

The base of transistor 70 in control circuit 66 is coupled to the outputof direction sensor circuit DS through resistor 70 and a diode 80, poledas shown. Transistors 70, 72 have their emitters coupled to ground andtheir collectors respectively connected through resistors 82 and 84 tothe C+ voltage supply source. The collector of transistor 70 isconnected to the base of transistor 72 and the ;ollector of transistor72 is connected through a resistor 86 to the base of transistor 70through diode 80. The collector of transistor 72 is also con nectedthrough a resistor 88 to the base of an NPN transistor 90, having itsemitter connected to ground and its collector connected in seriescircuit with relay coil CR4-C, which is connected in parallel with adiode 92, poled as shown.

PNP transistor 74 in circuit 68 has its base connected to the junctionof diode and resistor 86, and its collector connected through a resistor4 to the C voltage supply source. NPN transistor 76 has its emitterconnected to the emitter of transistor 74 and its base connected toground. The collector of transistor 76 is connected to the base oftransistor 74 through a resistor 96, as well as to the C+ voltage supplysource through a resistor 98. Transistor 78 has its base connected tothe collector of transistor 76 and its emitter connected to ground. Thecollector of transistor 78 is connected through a resistor to the C+voltage supply source, as well as through a resistor 102 to the base ofan NPN transistor 104. Transistor 104 has its emitter connected toground and its collector connected in series with relay coil CR3C, whichis connected in parallel with a diode 106, poled as shown.

The reverse control circuit 66 has an output circuit X taken from thecollector of transistor 72. Similarly, forward control circuit 68 has anoutput circuit Y taken from the collector of transistor 78. As will beappreciated from the description of operation presented hereinafter,when the vehicle is stationary, transistors 72 and 78 are conductiveand, consequently, the potential existing at circuits X and Y isessentially that of ground potential.

MOTION SENSING CIRCUIT The motion-sensing circuit MS, as shown in FIG.3, generally includes a pair of NPN transistors I and 112, together witha switch control means in the form of relay coil CR2-C. Transistor 110has its collector connected to the base of transistor 112 as well asthrough a resistor 114 to the C+ voltage supply source. The base oftransistor 110 is connected through a resistor 116 to the outputcircuits X and Y in the current direction control circuit CD throughrespective diodes 118 and 120, poled as shown. The base of transistor110 is also connected through resistor 116 to output circuit 2 of aninverter circuit l through a diode 122, poled as shown. The potentialexisting at output circuit Z will be essentially that of groundpotential whenever limit switch LS-l is in its closed position, as uponactuation of brake member 20. Relay coil C R2-C is connected between thecollector of transistor 112 and the 13+ voltage supply source through aresistor 124. A diode 126, poled as shown, is connected in parallel withrelay coil CR2C, and a capacitor 128 is connected between ground and thejunction of resistor 124 and relay coil CR2- C.

MISCELLANEOUS CIRCUITRY AND STRUCTURAL COMPONENTS Normally open limitswitch LS-l is coupled between the B+ voltage supply source and invertercircuit I. The inverter circuit l includes an NPN transistor 130, havingits emitter connected to ground and its collector connected to outputcircuit Z as well as through a resistor 132 to the C+ voltage supplysource. A diode 134, poled as shown, is coupled between the base andemitter of the transistor. The base of transistor 130 is also connectedthrough a resistor 136 to limit switch LS-l.

Limit switch LS1 is also connected between the B+ voltage supply sourceand an electrical braking system mode control switching means in theform of relay coil CRl-C through a resistor 140. The other end of relaycoil CRl-C is connected to the B- voltage supply source. A diode 142,poled as shown, is connected in parallel with the relay coil. Relay coilCR1C serves to control the operation of various switches, in the form ofrelay contacts, including normally closed relay contacts CRl-l forconnecting an acceleration potentiometer 42 to the control input ofpulse control circuit PC, as well as normally open relay contacts CRl-Zfor coupling the electrical brake potentiometer 28 to the control inputof pulse control circuit PC. Relay coil CR1C also controls the operationof normally open relay contacts CRI3 interposed between pulse stretcherPS and field winding 30, as well as normally closed relay contacts CR1-4interposed between the B voltage source and field winding 30.

It is contemplated that the invention use many of the componentstypically used with an electrical motor-driven vehicle, Consequently,vehicle A is provided with control circuitry for reversing field windingwith respect to the direction ofcurrent flowing from pulse controlcircuit PS. In the interests of simplifying the description of thisinvention, a simple relay control circuit is provided for this purpose.The circuit includes a pair of relays including relay coil CRS-Ctogether with its normally open contacts CRS-l and normally closedcontacts CRS-Z. The second relay includes relay coil CR6-C together withits normally open contacts CR6-l and normally closed contacts CR62.These relay contacts are arranged about field winding 30, as shown inFIG. 3a. Ifdirect current is to flow in the direction of arrow R, thenrelay coil CRS-C is energized, as by closing a normally open switch 5-]interposed between the coil and the 13+ voltage supply source. If, onthe other hand, current is to flow through field winding 30 in theforward direction, as indicated by arrow F, then coil CR6-C is energizedas by closing a normally open switch S-2 interposed between relay coilCR6C and the B+ voltage supply source.

In utilizing relay coils CRS-C and CR6-C, the present invention providesadditional control circuitry, such as normally closed relay contactsCR15 and normally open contacts CR1-6. Relay contacts CRl-S permitnormally open switches 8-1 and 8-2 to be effective during normaloperation of the vehicle. Relay contacts CR1-6 are closed during thebraking mode, when limit switch LS-l is closed, so that the functions ofswitches S-1 and S-2 are performed by normally open relay contacts CR31and CR4-l, respectively. These relay contacts, in turn, are controlledby relay coils CR3C and CR4-C, respectively, located in the currentdirection control circuit CD.

The B+ voltage source is connected through normally closed relaycontacts CR2-l, normally open contacts CRl-7 and resistor 154 to relaycoils CR4-C and CR3-C. As will be described in greater detailhereinafter, once relay CR2-C is energized, which takes place upon theconcurrent conditions that limit switch LS-l is closed and that outputcircuits X and Y of current control circuit CD are at ground potential,then relay contacts CR2-1 will open so as to deactivate the currentdirection control circuit CD. This will prevent energization of relaycoils CR4-C and CR3-C.

Typically, the electrically powered vehicles of the type describedherein are provided with a deadman switch. The switch is normally in anopen condition so that a mechanical brake, such as brake 36 shown inFIG. 3a, is resiliently biased into braking engagement with thevehicle's wheel 34. To obtain brake release, an operator must maintainthe deadman switch closed. This may be accomplished in various forms andfor purposes of simplifying the description of this invention, FIG. 3aillustrates a normally open deadman switch 5-3 coupled between the 13+voltage supply source and the coil of a solenoid 162. The solenoid mayinclude a plunger 164, which, upon closure of deadman switch 5-3, isdriven in the direction indicated by the arrow 166 and, through suitableinterconnection with brake shoe 38, serves to release the brake fromengagement with wheel 34. In accordance with the present invention, itis contemplated that during the operation of the vehicle, the deadmanswitch S-3 be closed and, con sequently, the brake is released. If brakemember 20 is depressed to its second position, as shown by the dottedlines in FIG. 2, normally open limit switch LS-2 will be opened. Thepurpose of this switch is to cause brake 36 to engage wheel 34 once theswitch is opened. Consequently, limit switch LS-2 is shown in FIG. 3 asbeing connected in series with deadman switch S-3 between the B+ voltagesupply source and solenoid coil 160. It is also the purpose of thepresent invention to release brake 36 whenever the vehicle is motionlessand brake member 20 is depressed. As previously described, under theseconditions relay coil CR2C becomes energized. To obtain this brakingfunction, normally closed relay contacts CR2-2 are connected in theseries circuit with limit switch LS-2 and deadman switch S-3.

OPERATION During normal vehicle operation brake member 20 is in itsnormal position, as shown by the solidlines in FIG. 2, and consequently,limit switch LS-l (see FIG. 3) is in its open position. Thus, relay coilCRl-C is not energized and accelerator potentiometer 42 is coupled tothe control input of pulse control circuit PC for controlling thevelocity of vehicle A. Vehicle direction control is accomplished byclosing one of n0rmally open switches 5-2 or Sl for controlling thecurrent flowing through field winding 30 in a forward direction orreverse direction, respectively. If a deadman switch, such as switch S3,is utilized by the vehicle, the switch will be in a closed conditionand, consequently, solenoid 162 is energized to release mechanical brake36.

If limit switch LS-l is closed while the vehicle is in a stationarycondition, then the potential existing at output circuits X, Y and Zwill be essentially that of ground potential. Thus, upon closure oflimit switch LS-l, transistor 130 is biased into conduction so that thepotential existing at the collector thereof, and hence at output circuitZ, is essentially that of ground potential, Similarly, since the vehicleis stationary, transistor 70, in circuit 66, is reverse biased causingtransistor 72 to be biased into conduction so that the potential at itscollector, and hence at output circuit X, is at essentially groundpotential. Also, with the vehicle in a stationary condition, transistors74 and 76, in circuit 68, are reverse biased so that transistor 78 isbiased into conduction and the potential exist ing at its collector, andhence on output circuit Y, is essentially at ground potential. Withground potential applied to the anodes of diodes 118, 120 and 122.transistor 110 in the motion sensing circuit MS is reverse biased sothat transistor 112 is biased into conduction to energize relay coilCRZ-C. When relay coil CR2-C is energized, its normally closed contactsCR2-1 are open to deactivate the current direction control circuit CD,and its normally closed contacts CR2-2 are also opened to therebydeenergize solenoid 162 so that brake 36 is biased into engagement withwheel 34 by spring 40. It will be noted from the circuitry of FIG. 30that if power is lost from the B+ voltage supply source, solenoid 162will be deenergized to actuate mechanical brake 36 into engagement withwheel 34.

If vehicle A is driven by motor M in either a forward or reversedirection and the operator actuates brake member 20 from its normalfirst position toward its second position, limit switch LS-l will beclosed to energize relay coil CRl-C Consequently, all of the relaysnormally open contacts will become closed and its normally closedcontacts will become open. The pulse frequency of the current pulsesprovided by the pulse control circuit PC will be controlled by theadjust ment of brake potentiometer 28 and these pulses will be stretchedby pulse stretcher PS before application to field winding 30. As brakemember 20 is further depressed toward its second position, the frequencyof these pulses will increase and this will determine the speed of motorM. Also, the control for field winding 30 is switched to normally openrelay contacts CR31 and CR4-l, and these relay contacts are, in turn,respectively controlled by circuits 68 and 66.

The direction of vehicle movement is sensed by tachometer 50 anddirection sensing circuit DS. if the vehicle is traveling in a forwarddirection at the time the operator actuates brake member 20, then theoutput signal of direction sensing circuit D8 will be of negativepolarity. In such case, this signal will bias transistor 74 and, hence,transistor 76 of circuit 68, into conduction. With transistor 76 beingbiased into conduction, transistor 78 will be reverse biased, whereupontransistor 104 is biased into conduction, With transistor 104 biasedinto conduction, relay coil CR3-C will be energized, causing itsnormally open contacts CR3-l to close. With contacts CR3-1 in a closedcondition, relay coil CRS-C will be energized to close its normally opencontacts CR] and open its normally closed contacts CR5-2. Consequently,the direct current pulses from pulse stretcher PS will flow throughfield winding 30 in the reverse direction as indicated by arrow R, Asthe frequency of the pulses is increased, the dynamic braking ac tion ofmotor M will be increased to decelerate the motor and, hence, vehicle Asubstantially in proportion to the position of brake member 20.

if in the above example, the vehicle was traveling in a reversedirection at the time the operator actuated brake member 20, then theoutput signal obtained from the direction sensing circuit DS would havebeen of positive polarity. ln such case, the signal would forward biastransistor 70, in circuit 66, into conduction. Since the collectorpotential of transistor 70 will be essentially that of ground potential,transistor 72 will become reverse biased to thereby forward biastransistor 90 into conduction. With transistor 90 being forward biasedinto conduction. relay coil CR4-C will be energized to close itsnormally open contacts CR41 to, in turn, energize relay coil CR6C. Withrelay coil CR6C being energized, its normally open contacts CR61 willbecome closed and its normally closed contacts CR62 will become opened.Consequently, the current pulses from pulse stretcher PS will flowthrough field winding 30 in a forward direction, indicated by arrow F,and thereby dynamically brake motor M in proportion to the pulsefrequency.

if, during the electrical braking operation, the operator requires apanic stop and depresses brake member 20 to its second position, limitswitch LS-2 will be opened. This will deenergize solenoid 162 so thatbrake 36 is resiliently biased by spring 40 into braking engagement withwheel 34.

Although the invention has been described and shown in conjunction witha preferred embodiment, it will be readily apparent to those skilled inthe art that various changes in form and arrangement of parts may bemade to suit requiremerits without departing from the spirit and scopeof the invention as defined by the appended claims.

Having thus described my invention, 1 claim: 1. An electrical brakingsystem for a vehicle driven by an electric motor having a field windingarranged so that said vehicle is driven in a direction in accordancewith the direction of direct current flowing through said field winding,said system adapted to be carried by a said vehicle and com prising:

a movable brake member adapted to be mounted to a said vehicle formovement between a normal first position and a second position;

means for varying the amount of direct current applied to said fieldwinding in dependence upon the relative position of said brake memberbetween said first and second positions;

means for sensing whether said vehicle is traveling either in first orsecond opposite directions; and,

current direction control means controlling the direction of saiddirect-current through said field winding so as to flow in a directionopposite to that for driving said vehicle in the sensed direction,whereby said electrical motor is electrically braked to retard motion ofsaid vehicle in said sensed direction substantially in proportion tosaid rela tive position of said brake member.

2. An electrical braking system as set forth in claim 1, wherein saidcurrent varying means includes a potentiometer having a wiper armmechanically coupled to said brake member for movement therewith.

3. An electrical braking system as set forth in claim 2, including:

first actuatable switching means having a normal first condition whereinsaid potentiometer is deactivated and a second condition wherein saidpotentiometer is activated; and,

switch control means for, upon energization, actuating said firstactuatable switching means to said second condition.

4, An electrical braking system as set forth in claim 3, including limitswitch means having a normal first condition and a second condition,when said brake member is moved from its normal first position towardsaid second position, for ener gizing said control means.

5. An electrical braking system as set forth in claim 1, includingactuatable switching means having a normal first condition foractivating said current direction control means and a second conditionfor deactivating said current direction control means; and, switchcontrol means for, upon energization, actuating said actuatableswitching means to said second con dition.

6. An electrical braking system as set forth in claim 5, includingcircuit means for energizing said switch control means only upon theconcurrent conditions that said vehicle is not moving and said brakemember is displaced from its first position toward said second position,whereupon said current direction control means becomes deactivated.

7 An electrical braking system as set forth in claim 6, includingmechanical brake control means for, upon energization of said switchcontrol means, controlling mechanical braking means to mechanicallybrake said vehicle.

8. An electrical braking system as set forth in claim 1, includingswitching means operated by said brake member for controlling mechanicalbraking means to mechanically brake said vehicle when said brake memberis displaced to its second position.

9. An electrical braking system as set forth in claim 1, wherein saidcurrent direction control means includes a forward control circuithaving first field current control means for, upon energization,controlling the direction of direct current flow in a forward directionthrough said field winding; and,

a reverse control circuit having a second field current control meansfor, upon energization, controlling the direction of said current flowin a reverse direction through said field winding.

10. An electrical braking system as set forth in claim 9, wherein saidforward control circuit includes first circuit means for energizing saidfirst field current control means when said vehicle is traveling in saidsecond direction and said reverse control circuit includes secondcircuit means for energizing said second field current control meanswhen said vehicle is traveling in said first direction.

11. An electrical braking system for a vehicle driven by an electricmotor having a field winding arranged so that said motor may be drivenin one of two opposite directions in accordance with the direction ofdirect current pulses flowing through said winding and at a velocitysubstantially proportional to the frequency of said pulses applied tosaid winding under the control of a pulse control circuit for changingsaid pulse frequency in dependence upon the value of a resistor coupledto an input of said circuit, said braking system adapted to be carriedby said vehicle and comprising:

a variable resistor adapted to be coupled between a direct currentvoltage source and said circuit input for varying said pulse frequency;

a movable brake member adapted to be mounted to said vehicle formovement between a normal first position and a second position, saidbrake member being coupled to said variable resistor for changing theresistance thereof in accordance with the relative position of saidmember as it is being displaced from said first position to said secondposition and thereby vary said pulse frequency;

means for sensing whether said vehicle is traveling in either first orsecond opposite directions; and,

current direction control means controlled by said sensing means forcontrolling the direction of said current pulses through said fieldwinding so as to flow in a direction opposite to that for driving saidvehicle in said sensed direction, whereby said electric motor iselectrically braked to retard motion of said vehicle in said senseddirection in accordance with said relative position of said brakemember.

12. An electrical braking system as set forth in claim ll, includingpulse stretcher means for increasing the pulse width of said currentpulses applied to said field winding to obtain greater electricalbraking forces for a given pulse frequency.

ill

13. An electrical braking system as set forth in claim 12, includingactuatable switching means having a normal first condition wherein saidpulse stretcher means is deactivated and a second condition wherein saidpulse stretcher means is activated; and

switch control means for, upon energization, actuating said actuatableswitching means to said second condition.

14. An electrical braking system as set forth in claim i3, includinglimit switch means having a normal first condition and a secondcondition, when said brake member is moved from its normal firstposition toward said second position, for energizing said switch controlmeans and, hence, activating said pulse stretcher means. I

15. An electrical braking system as set forth in cla1rn ll,

wherein said variable resistor has a wiper arm coupled to said brakemember for movement therewith to vary the resistance of said variableresistor.

16. In an order picker vehicle driven by a direct current operatedelectric motor having a field winding, said vehicle having a liftvertically movable between lower and upper positions, said lift havingmeans for supporting an operator together with manually operablecontrols for controlling lift height and vehicle speed, the improvementfor electrically braking said vehicle comprising:

a manually movable brake member mounted to said lift for movementrelative to said lift between a normal first position and a secondposition;

means for varying the amount of direct current applied to said fieldwinding in dependence upon the relative position of said brake memberbetween said first and second positions;

vehicle direction sensing means for sensing whether said vehicle istraveling in a forward or reverse direction; and,

motor current control circuit means controlled by said direction sensingmeans for reversing the direction of direct current flowing through saidfield winding to dynamically brake said motor and thereby retard motionof said vehicle in said sensed direction with a braking force dependenton the relative position of said brake member between its said first andsecond positions.

17. In an order picker vehicle as set forth in claim 16, wherein saidcurrent varying means includes a variable resistor having a wiper armcoupled to said brake member for movement therewith.

18. in an order picker vehicle as set forth in claim 16, including firstlimit switch means mounted to said lift so as to be actuated by saidbrake member only when said brake member is displaced from its firstposition toward said second position, and

circuit control means for deactivating said motor current controlcircuit means only upon the concurrent condi tions that said first limitswitch means is actuated and said vehicle is stationary.

19. in an order picker vehicle as set forth in claim 16, wherein saidvehicle has actuatable mechanical braking means and said improvementfurther includes:

second limit switch means mounted to said lift so as to be actuated whensaid brake member is displaced to its second position for, in turn,actuating said mechanical braking means.

1. An electrical braking system for a vehicle driven by an electricmotor having a field winding arranged so that said vehicle is driven ina direction in accordance with the direction of direct current flowingthrough said field winding, said system adapted to be carried by a saidvehicle and comprising: a movable brake member adapted to be mounted toa said vehicle for movement between a normal first position and a secondposition; means for varying the amount of direct current applied to saidfield winding in dependence upon the relative position of said brakemember between said first and second positions; means for sensingwhether said vehicle is traveling either in first or second oppositedirections; and, current direction control means controlling thedirection of said direct current through said field winding so as toflow in a direction opposite to that for driving said vehicle in thesensed direction, whereby said electrical motor is electrically brakedto retard motion of said vehicle in said sensed direction substantiallyin proportion to said relative position of said brake member.
 2. Anelectrical braking system as set forth in claim 1, wherein said currentvarying means includes a potentiometer having a wiper arm mechanicallycoupled to said brake member for movement therewith.
 3. An electricalbraking system as set forth in claim 2, including: first actuatableswitching means having a normal first condition wherein saidpotentiometer is deactivated and a second condition wherein saidpotentiometer is activated; and, switch control means for, uponenergization, actuating said first actuatable switching means to saidsecond condition.
 4. An electrical braking system as set forth in claim3, including limit switch means having a normal first condition and asecond condition, when said brake member is moved from its normal firstposition toward said second position, for energizing said control means.5. An electrical braking system as set forth in claim 1, includingactuatable switching means having a normal first condition foractivating said current direction control means and a second conditionfor deactivating said current direction control means; and, switchcontrol means for, upon energization, actuating said actuatableswitching means to said second condition.
 6. An electrical brakingsystem as set forth in claim 5, including circuit means for energizingsaid switch control means only upon the concurrent conditions that saidvehicle is not moving and said brake member is displaced from its firstposition toward said second position, whereupon said current directioncontrol means becomes deactivated.
 7. An electrical braking system asset forth in claim 6, including mechanical brake control means for, uponenergization of said switch control means, controlling mechanicalbraking means to mEchanically brake said vehicle.
 8. An electricalbraking system as set forth in claim 1, including switching meansoperated by said brake member for controlling mechanical braking meansto mechanically brake said vehicle when said brake member is displacedto its second position.
 9. An electrical braking system as set forth inclaim 1, wherein said current direction control means includes a forwardcontrol circuit having first field current control means for, uponenergization, controlling the direction of direct current flow in aforward direction through said field winding; and, a reverse controlcircuit having a second field current control means for, uponenergization, controlling the direction of said current flow in areverse direction through said field winding.
 10. An electrical brakingsystem as set forth in claim 9, wherein said forward control circuitincludes first circuit means for energizing said first field currentcontrol means when said vehicle is traveling in said second directionand said reverse control circuit includes second circuit means forenergizing said second field current control means when said vehicle istraveling in said first direction.
 11. An electrical braking system fora vehicle driven by an electric motor having a field winding arranged sothat said motor may be driven in one of two opposite directions inaccordance with the direction of direct current pulses flowing throughsaid winding and at a velocity substantially proportional to thefrequency of said pulses applied to said winding under the control of apulse control circuit for changing said pulse frequency in dependenceupon the value of a resistor coupled to an input of said circuit, saidbraking system adapted to be carried by said vehicle and comprising: avariable resistor adapted to be coupled between a direct current voltagesource and said circuit input for varying said pulse frequency; amovable brake member adapted to be mounted to said vehicle for movementbetween a normal first position and a second position, said brake memberbeing coupled to said variable resistor for changing the resistancethereof in accordance with the relative position of said member as it isbeing displaced from said first position to said second position andthereby vary said pulse frequency; means for sensing whether saidvehicle is traveling in either first or second opposite directions; and,current direction control means controlled by said sensing means forcontrolling the direction of said current pulses through said fieldwinding so as to flow in a direction opposite to that for driving saidvehicle in said sensed direction, whereby said electric motor iselectrically braked to retard motion of said vehicle in said senseddirection in accordance with said relative position of said brakemember.
 12. An electrical braking system as set forth in claim 11,including pulse stretcher means for increasing the pulse width of saidcurrent pulses applied to said field winding to obtain greaterelectrical braking forces for a given pulse frequency.
 13. An electricalbraking system as set forth in claim 12, including actuatable switchingmeans having a normal first condition wherein said pulse stretcher meansis deactivated and a second condition wherein said pulse stretcher meansis activated; and switch control means for, upon energization, actuatingsaid actuatable switching means to said second condition.
 14. Anelectrical braking system as set forth in claim 13, including limitswitch means having a normal first condition and a second condition,when said brake member is moved from its normal first position towardsaid second position, for energizing said switch control means and,hence, activating said pulse stretcher means.
 15. An electrical brakingsystem as set forth in claim 11, wherein said variable resistor has awiper arm coupled to said brake member for movement therewith to varythe resistance of said variable resistor.
 16. In an order picker vehicledriven by a direct current operated electric motor having a fieldwinding, said vehicle having a lift vertically movable between lower andupper positions, said lift having means for supporting an operatortogether with manually operable controls for controlling lift height andvehicle speed, the improvement for electrically braking said vehiclecomprising: a manually movable brake member mounted to said lift formovement relative to said lift between a normal first position and asecond position; means for varying the amount of direct current appliedto said field winding in dependence upon the relative position of saidbrake member between said first and second positions; vehicle directionsensing means for sensing whether said vehicle is traveling in a forwardor reverse direction; and, motor current control circuit meanscontrolled by said direction sensing means for reversing the directionof direct current flowing through said field winding to dynamicallybrake said motor and thereby retard motion of said vehicle in saidsensed direction with a braking force dependent on the relative positionof said brake member between its said first and second positions.
 17. Inan order picker vehicle as set forth in claim 16, wherein said currentvarying means includes a variable resistor having a wiper arm coupled tosaid brake member for movement therewith.
 18. In an order picker vehicleas set forth in claim 16, including first limit switch means mounted tosaid lift so as to be actuated by said brake member only when said brakemember is displaced from its first position toward said second position,and circuit control means for deactivating said motor current controlcircuit means only upon the concurrent conditions that said first limitswitch means is actuated and said vehicle is stationary.
 19. In an orderpicker vehicle as set forth in claim 16, wherein said vehicle hasactuatable mechanical braking means and said improvement furtherincludes: second limit switch means mounted to said lift so as to beactuated when said brake member is displaced to its second position for,in turn, actuating said mechanical braking means.